Photographic silver halide emulsions



Patented May 5, 1942 Wesley G. Lowe,

Eastman Kodak EMULSIONS Rochester, N. Y., assignor to Company, Rochester, Y., a

corporation oi New Jersey No Drawing. Application August 1,

Serial No. 349,294

60laims.

This invention relates to a methodoi making photographic silver halide emulsions in which the silver halide is first prepared in an aqueous solution of a silver halide dispersing agent and the dispersion is subsequently thoroughly mixed with a polyvinyl acetaldehyde acetal having a polyvinyl. acetal content of at least50% and an uncombined hydroiwl content corresponding to at least 15% polyvinyl alcohol and the emulsion prepared thereby. 4

In U. 8. Patent No. 2,211,323, dated August 13, 1940, Charles R. Fordyce, the use of polyvinyl acetaldehyde acetal resins. oi the polyvinyl acetal content and hydroxyl content indicated as the carrier for silver halide in photographic emulsions, is described and claimed. Those photographic emulsions are quite satisiactory for the recording of images. i The present invention is an improvement thereover.

One object of my invention is to improve the permeability of photographic coatings containing polyvinyl acetal resins to water and to photographic baths. Another object o! my invention is to improve the speed or developing and fixing polyvinyl acetal resin-silver halide emulsions and to improve the contrast and sensitivity of those emulsions. Other objects of my invention will appear herein.

I have found that it is possible to improve the developing and fixing speed and the contrast and sensitivity 0! photographic silver halide emulsions in which a polyvinyl acetal is employed as the carrier when the silver halide is prepared and dispersed first in a silver halide dispersing agent prior to the addition of at least a substantial proportion of the polyvinyl acetal resin, which is employed as the carrier.

Ordinarily gelatin is employed as the carrier for the silver halide in photographic emulsions of this type. These emulsions are ordinarily prepared by first forming the silver halide in an aqueous solution of the gelatin. In the preparatlon of photographic silver halide emulsions in which a polyvinyl acetal resin is employed as the carrier, the same procedure has been followed, namely an aqueous solution of the acetal resin has been formed therein. I have found, however, that ii the silver halide is prepared in an aqueous solution of a material having good dispersing properties towards silver halide, the silver halide is produced in the form of fine grains and thus is more completely distributed in the emulsion which finally results. v

In accordance with my invention, the preparation of silver halide for the emulsion is carried out in a very dilute aqueous solution of a material which has. the power of preventing the agglomeration of the particles of silver halide when they form. It must therefore be soluble.

in either water alone or a mixture of water with an organic solvent. It must also have the power of holding the silver halide in suspension, must be compatible with the polyvinyl acetaldehyde acetal resin employed for the best results are unsuitable for this purpose while other materials not identified ordinarily: as dispersing agents are found to be useful in this connection.

One of the most suitable silver halide dispersing agents for acetaldehyde acetal emulsions is g the co-polymeroi maleic acid and vinyl acetate having an apparent molecular weight 01 the order at magnitude 3,000-200.000, based on viscosity measurements, as described int]. 8. Patent No. 2,047,398 of I. G. The condensation product oi this co-polymer with aniline, as de-,

scribed in the U. S. patent reierred to, has also been found to be useful for dispersing. silver halide in accordance with the process 01 my invention.

Some carbohydrate derivatives, having the properties'described above. may be regarded as silver halide dispersing agents such as the watersoluble alkyi celluloses. For example, the watersoluble methyl celluloses, having a methoxy content of 20-30%, are particularly suitable for this purpose. Other water-soluble lower alkyl celluloses. having a corresponding alkoxy content such as ethyl cellulose may be employed. The

water-soluble compound, may be an ethanol I amine cellulose acetate, such as diethanolamine cellulose acetate. The ethanolamine cellulose acetate'may be prepared by reacting upon cellulose acetate which has been hydrolyzed to a low acetyl content, such as 22%, with p-toluene sulionyl chloride and pyridine to form a cellulose acetate-p-toluene sulionate which is then reacted with an ethanolamine (either mono-, dior tri-). These compounds and their prepara-' tion are theinvention oi Kenyon and Reynolds and are described and claimed in their application Serial No. 349,222, filed of even date. Any. other materials having theproperties prescribed for a silver halide dispersing agent, which are compatible with a polyvinyl acetaldehyde acetal contain acetate groups;

emulsion, are suitable for use herein for that purpose.

The silver halide dispersing agents are employed in preparing silver halide in the form of dilute aqueous solutions. For instance, a dilution of the dispersing agent in a concentration within the range of '0.5-6% of dispersing agent is ordinarily employed for the preparation of the silver halide. It is to be understood, however, that the proportion of silver halide dispersing agent may vary in accordance with the judgment of the individual operator. Forenample, if greater contrast is desired in the, photographic emulsion, the proportion of dispersing agent used may be increased.

It may be stated as a general rule that the ratio of silver halide dispersing agent used to the polyvinyl acetal resin employed, as the protective coating, should be within the range of approximately 0.5-4:6 (by weight). For example, in the case of water-soluble methyl cellulose, the ratio to polyvinyl acetal resin of approximately 1:5 may be used although the addition of some solvent, such as ethyl alcohol, will aid the dispersing power'in which case a lower ratio, such as 1:10, will give'good results. In some cases it may be desired to mix two or more'dispersing agents.-

sensitizing dye. It may then be coated on a support and dried with a current of warm air to form a hard, scratch-resistant, light-sensitive emulsion layer. It may then be exposed, processed, washed and dried and good prints will result. 7

These photographic silver halide emulsions may be sensitized in thesame manner as are silver halide emulsions generally. Sensltizers, which have been previously employed to optically sensitize silver halide emulsions, may be incorporated in the emulsions described herein to increase Brooker, dated July 11, 1939.

' tion:

Also, the mixing in of a little polyvinyl acetal resin with the dispersing agent is not objectionable. However; the presence of the resin in the preparation of the silver halide, when using a dispersing agent, appearsto serve no useful purpose and, therefore it is preferred that the preparation of the halide be carried out in the substantial absence of the resin which is subsequently employed as the carrier. e

As pointed out above, the polyvinyl acetaldehyde acetal resins, having a polyvinyl acetal content of at least 50% and ahydroxyl content corresponding to at least 15% of polyvinyl alcohol,

have been found to be particularly useful as the vehicle or protective colloid for the dispersed silver halide. It is characteristic of these resins that they are soluble in coldwater or in water mixed with an organic solvent. Therefore, in making up the emulsion it is ordinarily desirable that either a lower temperature be used or a solvent be employed with the water. The resins, which are used for a carrier, will always contain acetal and hydroxyl groups and they may or may not The acetal portion of the compound may be composed entirely of acetaldehyde or there may be some other aldehyde mixed therewith, such as formaldehyde or' tion of a polyvinyl acetal resin, as described, is-

then added and thoroughly incorporated in the mass, such as by stirring the materials together for a few minutes. Unless cold conditions are present, a small amount of a water-miscible solvent, such as acetone, ethyl alcohol, methyl or ethyl cellosolve is mixed with the water in forming the aqueous solution of the acetal resin. The mass, which may be termed an emulsion, with or Without warming followed by cooling, may, if desired, be sensitized with a small amount of a Example I A solution of gram of cellulose methyl ether having a methoxyl content of approximately 28% After the preparation of the silver halide in the dispersing agent was completed, 20 cc. of acetone was added followed by the addition; of 45 grams of a 14% solution of polyvinyl acetaldee hyde acetal containing 31.7% polyvinyl alcohol in ethanol-water (3:2). The emulsion was diluted with 20 cc. of acetone and 15. cc. of ethyl cellosolve. 2 mg. of 2,3-diethyl-4'-methyloxathiazolocarbocyanine iodide .was' incorporated therein to sensitize the emulsion and the emulsion was coated on paper. The photographic paper formed after the customary processing was found to give prints of good quality.

Example I] 2 grams of the co-polymer of maleic acid and vinyl acetate was dissolved in 50 cc. of water and heated to F. 7 Solutions A and B, as described in the preceding example, were added thereto simultaneously over a period of three minutes with stirring. 40 grams of a 14%. solution of polyvinyl acetal, containing 32% ofvinyl al-' cohol, in ethanol water (2:3) was added and also 25cc. of acetone was added. The emulsion was sensitized by thoroughly incorporating therewith 2 mg. of 2,3'-diethyl-4-methyloxathlazolocarbocyanine iodide and coated on paper. The

emulsion was dried and the photographic paper. was processed in the usual baths. It was found that the time of developing and fixing was more rapid than with emulsions where .acetal was used alone and the sensitivity of the emulsion was greater.

1' claim:

1. A method of preparing a photographic sil-. ver halide emulsion which comprises preparing a silver halide in an aqueous solution of a silver halide dispersing agent and subsequently adding thereto a polyvinyl acetaldehyde acetal resin having a polyvinyl acetal content of at least 50% and an uncombined hydroxyl content corresponding to at least 15% polyvinyl alcohol.

The following examples illustrate'my inven-i 2. A method 0! preparing a photographic silver halide emulsion which comprises preparing a silver halide in an aqueous solution of a silver halide dispersing agent and subsequently adding thereto an optical sensitizer and as a protective colloid, a polyvinyl acetaldehyde acetal having a polyvinyl acetal content of at least 50% and an uncombined hydroxyl content corresponding to at least 15% polyvinyl alcohol.

3. A method of preparing a photographic silver halide emulsion which comprises preparing a silver halide in an aqueous solution of a watersoluble alkyl cellulose and subsequently adding thereto an aqueous solution of a polyvinyl acetaldehyde acetal having a polyvinyl acetal content of at least 50% and an uncombined hydroxyl content of approximately 30%.

4. A photographic silver halide emulsion essentially consisting of finely dispersed silver halide, a silver halide dispersing agent and as the protective colloid, a polyvinyl acetaldehyde acetal having a polyvinyl acetal content of at least 50% and an uncombined hydroxyl content corresponding to at least 15% polyvinyl alcohol, said emulsion resulting from the preparation of the silver halide in an aqueous dispersion of said silver halide dispersing agent and the subsequent addition thereto of said protective colloid.

5. A photographic silver halide emulsion essentially consisting of finely dispersed silver halide, a silver halide dispersing agent, a protective colloid essentially consisting of a polyvinyl acetaldehyde acetal having a polyvinyl acetal content 01 at least 50% and an uncombined hydroxyl content corresponding to at least 15% polyvinyl alcohol and an organic plasticizer for the emulsion, said emulsion resulting from the preparation of the silver halide in an aqueous dispersion of said silver halide dispersing agent and the subsequent addition thereto of said protective colloid.

6. A photographic silver halide emulsion essentially consisting of finely dispersed silver halide, a silver halide dispersing agent essentially consisting of water-soluble alkyl cellulose and as the protective colloid, a polyvinyl acetaldehyde acetal having a polyvinyl acetal content-of at least 50% and an uncombined hydroxyl content corresponding to at least 15% polyvinyl alcohol, said emulsion resulting from the preparation of the silver halide in an aqueous dispersion of said silver halide dispersing agent and the subsequent addition thereto of said protective colloid.

WESLEY G. LOWE. 

